A tubular implant structure is well known in, for instance, the field of intraluminally deliverable implants and is usually within a radial range and within an axial range expandable and contractable. A so-called “stent” is a well known example of such a structure. Such a structure is usually introduced into the body of a human or an animal and delivered to a site within that body where the tubularity of the structure can play a crucial role in the functioning of at least that part of the body. That part is usually a part of the vascular system of the body, but can also be a part of another system of the body, for instance the biliary system or the digestive system.
A catheter is often used for delivery at the implant site. Whilst being delivered, the diameter of the tubular implant structure is preferably as small as possible. Currently, the diameter can be as small as 6-5 French (three French being equal to 1 mm). Once arrived at the implant site, the tubular implant structure is usually made to expand so that its diameter reaches a diameter within a predetermined range, for instance 18-24 F. Given these transformations, it will be appreciated that the tubular implant structure is often a complex device comprising struts connected in a sophisticated manner so that the structure can perform the required “jobs”.
Referring again to the example of a stent, in the radially expanded condition, it provides a “scaffolding” that internally supports the inner wall of a lumen and helps keeping the lumen open so that the fluid flow through that lumen can continue. For the vascular system is the lumen part of a vessel and the fluid flow a blood flow.
As usually the part of the body in which the tubular implant structure has been implanted, may be subjected to various types of movement and a large number of these measurements, it is necessary to know in advance the behaviour of the implant structure when subjected to such movements and its behaviour after having been subjected to a number of such movements that may occur during the remaining lifetime of the body. In particular structures implanted in ligaments or in for instance the neck, may experience subjection to various movements, and consequently to deformations such as torsion, axial compression, axial elongation, bending, and combinations thereof.
US 2004/0016301 A1 describes a vascular prosthesis tester that is configured to subject a stent, possibly provided with a graft, to tensile and/or compressive axial loads, bending stresses, and torsional stresses. Actuators may induce the stresses independently or in combination, or in a manner to simulate physiological movement, such as walking. The test member, i.e. the stent or stent-graft, may be disposed upon the outer surface of a fluid conduit. It is also possible that the stent or stent-graft is contained within a channel of a fluid conduit. The stent or stent-graft is “friction fit” placed within or around the conduit. A fluid is injected into the central lumen of the conduit to subject the test member to stress as applied by a change of blood pressure in a vessel during the pumping of the heart. The axial movement to which the tubular implant structure, in this case the stent, is subjected during the test, depends very much on the mechanical properties of the combined structure of the tubular implant structure and the fluid conduit. It is most likely that the fluid conduit is less flexible than the tubular implant structure. In that case, the movement to which the tubular implant structure is subjected, is for a great deal dependent on the response of the fluid conduit to the movements imposed on the fluid conduit by the actuators. To impose any movement at the middle portion of the tubular implant structure, the actuators usually apply a large “stroke” to the positions of the fluid conduit which are far away from the middle portion of the tubular implant structure, and which are sometimes even far away from the ends of the tubular implant structure. The maximum “stroke” as controlled by the actuators, is then “passed on” from these controlled positions towards the middle portion of the fluid conduit and the middle portion of the tubular implant structure. A disadvantage of this method is thus that the middle portion of the tubular implant structure is relatively little subjected to movement. The outer portion of the tubular implant structure, the ends of the stent in this case, are subjected to relatively large movement. The larger the movement, the larger the deformation and the smaller the movement, the smaller the deformation. It follows that the smaller deformation occurs at the middle portion of the tubular implant structure. Fracture is likely to occur at the outer ends of the tubular implant structure rather than at the middle portion. As a result of this, it is not the design of the tubular implant structure, in this case of the stent, which is being fatigue tested, but rather the end portions which suffer from all kinds of side effects. The outcome of the test does not allow for comparing two differently designed tubular implant structures, as the response of the ends of the tubular implant structure dominate the fracture behaviour. In general make a fair comparison of fatigue behaviour between differently designed implant structures very troublesome.
It is an object of one embodiment of the invention to provide an implant holder for holding a substantially tubular implant structure during fatigue testing of the implant structure such that the middle portion of the tubular implant structure is subjected to the maximum movement rather than the outer portions of the tubular implant structure.
It is an object of one embodiment of the invention to provide an implant holder which allows for a high reproducibility of fatigue tests.
It is an object of one embodiment of the invention to provide an implant holder which allows for a fair comparison of two differently designed tubular implant structures.
It is a further object of one embodiment of the invention to provide a holder which allows for connecting a tubular implant structure to a fatigue test system so that the connection itself is unlikely to contribute to the damage the tubular implant structure experiences during fatigue testing.